1. Field
This invention relates generally to a semiconductor circuit including one or more semiconductor devices, where each device is liquid cooled and, more particularly, to a semiconductor circuit including a plurality of GaN semiconductor devices fabricated on a common substrate, where the substrate includes a diamond heat removal layer formed in a microchannel below each device, and where a manifold is mounted to the substrate that includes impingement jets that spray a liquid cooling fluid onto the diamond layers.
2. Discussion
Integrated circuits are typically fabricated by epitaxial fabrication processes that deposit or grow various semiconductor layers on a wafer substrate to provide the circuit components for the device. Substrates for integrated circuits can include various materials, usually semiconductor materials, such as silicon, sapphire, SiC, InP, GaAs, etc. As integrated circuit fabrication techniques advance and become more complex, more circuit components are able to be fabricated on the substrate within the same area and be more closely spaced together. Further, these integrated circuit fabrication techniques allow the operating frequencies of the circuit to increase to very high frequencies, well into the GHz range.
Virtually all electronic components operate in a thermally limited capacity, that is, the performance of the device is limited by the amount of heat that can be dissipated from the device to the environment. The amount of thermal dissipation is proportional to the operating voltages, currents and frequencies of the device, where any increase results in higher power dissipation and thus waste heat. The rise of the electronic device junction temperature in the device is proportional to the thermal resistance between the device channel or junctions and the point at which the heat is released to the environment. Every device has a maximum junction temperature, where operation of the device beyond that temperature results in diminished performance and reliability due to basic limitations of the semiconductor and packaging materials. The desire to operate at higher powers (voltage, current, and/or frequency) drives the need for a reduction in thermal resistance.
One example of these types of devices includes gallium nitride (GaN)-based RF and microwave power amplifiers. GaN is a wide bandgap semiconductor and GaN-based high electron mobility transistors (HEMTs) have the ability to operate at both high current and high voltage. This type of operation coupled with fine geometries results in megawatt per square centimeter (MW/cm2) power densities near the gate finger of the device. GaN HEMT devices are typically epitaxially grown on a suitable substrate for these applications, where the substrate needs to be highly thermally conductive, electrically insulative, have a thermal expansion coefficient similar to GaN and provide lattice constant matching for suitable epitaxial growth. Suitable materials that are both highly thermally conductive and electrically insulative are relatively unique.
A high thermally conductive substrate for these devices is necessary so that heat is removed from the device junction through the epitaxial layers and the substrate so that the device is able to operate at high power in a reliable manner. Particularly, as mentioned above, as the temperature of the device increases above some threshold temperature, the electrical performance of the device is reduced, which reduces its high power capability. Further, too high of a temperature within the device reduces its reliability because its time to failure will be reduced. Also, these types of devices are typically high frequency devices, which become smaller in size, which reduces the spacing between gate fingers and device unit cells, as the frequency increases, which reduces their ability to withdraw heat. The conductive path for heat generated at the device junction layer in an HEMT device causes the heat to propagate through the epitaxial layers and the substrate and into the device packaging. Therefore, it is necessary to provide a high thermally conductive substrate that does not impede the path of the heat exiting the device, and allows the heat to spread out over a larger area. The thickness of the substrate is optimized to provide a low resistance heat path into the packaging from the device and provide the ability to spread the heat out away from the device.
For GaN HEMT devices, silicon carbide (SiC) substrates are currently the industry standard for providing the desirable characteristics of electrically insulating, highly thermally conductive, a close lattice match to that of GaN and a similar thermal expansion coefficient to that of GaN. SiC has a relatively high thermal conductivity, but the power dissipation is still limited by thermal constraints and the devices are not allowed to perform at their maximum levels. Although SiC is a good thermal conductor, its thermal conductivity is still limited, and as the junction temperature rises in the device, the ability of the SiC substrate and the heat sink to remove heat is limited, which limits the output power of GaN HEMT devices, and subsequently their reliability, as discussed above.
It is desirable to provide a suitable substrate for a GaN HEMT device that has a greater thermal conductivity than SiC. Diamond is electrically insulating and has the highest thermal conductivity of any bulk material. However, it is currently not possible to epitaxially grow GaN layers on large area single-crystal diamond substrates for many reasons, including availability, a large lattice constant mismatch and different thermal expansion coefficients.
Efforts have been made in the industry to overcome these problems so as to use diamond substrates in semiconductor devices, such as GaN HEMT devices. Diamond thermal vias have been previously conceived to improve the thermal resistance of semiconductor substrates by bringing high thermal conductivity diamond conduits close to the device active area where the heat is most concentrated.
U.S. Pat. No. 8,575,657 issued to Gambin et al. discloses a GaN HEMT device having a silicon carbide substrate including a via formed therein, where a diamond layer is provided within the via to dissipate heat generated by the device.
For certain semiconductor circuit applications, such as for high power amplifiers, GaN HEMT devices are employed in combination with various RF devices, waveguides, transmission lines, etc., where the HEMT devices provide a concentrated heat source in the circuit that needs to be spread out and directed to a heat sink typically at the bottom of the circuit. These types of GaN HEMT devices in monolithic microwave integrated circuit (MMIC) power amplifiers generate heat at a very high level where the maximum power density and performance of the devices can increase with the capability to dissipate additional heat. Further, mutual heating affects of neighboring device cells limit the GaN MMIC compaction which would otherwise lead to cost savings and performance improvements. For heat densities generated by these devices, the ability of SiC substrates and bulk diamond heat removal vias is often not enough to remove the desired amount of heat. In certain circuit designs, it is possible to place heat sinks in direct contact with the diamond layer so as to affectively remove heat therefrom for these types of applications. However, in some circuit designs, where other circuit components need to be placed where the heat sink is configured require the heat sink to be removed some distance from the diamond layer.